February 1, 1993
RICHARD L. COLLINS
As we contemplate the winter season from the dead of it, it is easy to see why some years ago light airplane flying used to virtually cease in the winter. There was local flying on good days, but that was about it. Then came somewhat faster airplanes, with heaters that worked, better equipment for instrument flying, and deice gear for some. The instrument flying was a big part of it, too, because weather systems are plentiful in winter, and there are not a lot of days when you can avoid all of them on a longer trip.
Actually, if you have an airplane that will cruise at 140 knots or better, making those long trips in the winter is not that much different than at other times of the year. The biggest difference is in the effort it takes to get the airplane ready to fly. If you fly a 120-knotter, there can be some quite long days and evenings headed west behind a cold front or southwest in front of one.
For the instrument pilot, ice is the big consideration. Dealing with ice is not easy, either. Information on it is sparse at best because, except for pilot reports, what we get is opinion.
In searching for icing information, whether on DUAT or from a briefer, we deal primarily with the sigmet and airmet portion of the area forecast. Sigmets cover severe icing; airmets cover moderate icing. Severe means you can't deal with it even if equipped; moderate means even short encounters are potentially hazardous, and deice equipment or flight diversion is necessary.
Because the forecaster is charged with covering every eventuality over the area and in the time period, and area forecasts are prepared but three times a day, they paint with a broad brush. One DUAT briefing I ran for an example covers most of the eastern United States on a day when the Today show map says it will be clear. It calls for moderate rime or mixed icing in clouds and in precipitation from the surface to 7,000 feet. The airmet goes on to define where the freezing level might be. Basically, this illustrates that it is very difficult to fly IFR in the winter without trespassing in an area where there is an airmet covering potential icing.
We can at least get some clues from the airmet. The fact that this one put the top of the icing at 7,000 likely means that there are no clouds expected above that level anywhere in the area. The other clue this day is that the freezing level is expected to slope to 4,000 to 6,000 feet east of an Atlantic City/Greensboro line. (States are frequently left out in a telephone briefing, so if your geography isn't good, never fail to ask where a defining point is located — New Jersey and North Carolina in this case.)
The next thing is to make a plan that keeps the airplane away from ice, or that sends you to activities other than flying. One plan can be VFR beneath the clouds if the cloud bases and terrain allow. A good thing about that plan is that you can scurry to the nearest airport whenever conditions appear unfavorable for a continuance.
Most instrument-rated pilots would rather do it IFR, though, and a flight related to me by a pair of relatively new pilots illustrates how the planning can go awry.
If I recall the story of the adventure correctly, the cloud tops were reported at 7,000 feet. Given the time of year and the conditions, there was surely an airmet for icing, probably up to 7,000 feet. These pilots chose to file for 9,000 feet, which would be on top and free of the ice.
If you have read this far, you are going to guess what happened next. They were cleared to 5,000 feet, where the airframe started accumulating ice. What to do next? They had filed and flown into an area where there was ice, and once it started accumulating, their flight conditions were definitely in "known" ice. (The word is in quotes because while it has been bandied about for years, it has never been defined. Many icing flight approvals refer simply to "icing conditions.") Their plan was to climb through quickly and accumulate little of the stuff; their reality was the interface with an air traffic control system that had another idea. What to do?
The mandate when ice starts to accumulate is to get out of it. These pilots were dedicated to that principle, but not having flown in icing conditions many times, they really weren't sure what to do next. Declare an emergency, and force the controller to move other airplanes and approve a climb? That would work, but it also would give the Federal Aviation Administration prima facie evidence of intentional flight into icing conditions, a no-no. These pilots were not aware of the possible consequences of using the emergency authority of the pilot in command in such conditions. Perhaps they could have told the controller that they would take any new routing or a heading that would allow an immediate climb. If that didn't work, then go to plan E, as in the word. Happily for them, they were but a few minutes and a little ice into the condition when the controller cleared them to 9,000 feet, and the problem was solved.
The legality of filing up through an icing layer to an on-top condition is questionable. The FAA would, if there was any sort of an incident, likely go after you with a vengeance. Pilots do it, though. Clearances to maintain a lower than filed altitude for a while are common, especially in busy areas. That should be considered. The cloud tops can slope upward, especially if you are flying toward a low pressure or frontal system, toward higher terrain, or toward the Great Lakes. That should also be considered. A pilot report giving cloud tops is the only way you can get the information, and the report's age and the location of the reporter should be considered in relation to the other conditions just mentioned.
One ace in the hole often presents itself here. If the surface and the maneuvering altitudes for an approach are above freezing, a return to the departure airport and landing is often a solution, and such requests are usually handled quickly.
If ice is forecast but there are no pilot reports of ice, does that suggest that it is okay to fly into an area where icing is covered by an airmet? Consider the next example.
I was headed for Rochester, New York, which is on the lee side of the Great Lakes. The weather there was cloudy, with an approaching cold front that was supposed to pass through a bit later with rapidly improving conditions. The area forecast included moderate ice below 12,000 feet, and there were no pilot reports of ice. The weather for departure from Maryland was clear.
Flying an airplane with a service ceiling well above 12,000 feet, I would stay on top of the ice, and then see what it looked like near Rochester. In an unpressurized airplane, oxygen would have been required because, this being a westbound flight, 14,000 feet might become the on- top altitude. I found, in years of flying airplanes without turbochargers in the winter, that an oxygen supply in those airplanes was often the ticket to a much less eventful trip when ice was a question. I remember coaxing my Cardinal RG to 17,000 feet one day and finding that, one, it did pretty well up there except that, two, the heater was 100-percent inadequate. Make sure you have an oxygen supply, a woolly-bully coat, and some warm gloves.
Anyway, on the trip to Rochester, I tried to stay as low as possible because of strong headwinds. As the cloud tops rose in classic fashion, my altitude request approvals were slow coming through, and there was some ice on the unprotected surfaces of my deiced airplane as I neared the Rochester area. Some residual ice also stuck on the boots. Then the magic word came from an airline aircraft — moderate ice on the descent into Rochester. At this time, I was on top in clear air, and I made the decision to turn around and go home because of that report. Too much of a combination of bad things there.
On another trip across the Great Lakes, I found that it took Flight Level 180 to get on top of all clouds on the lee side of Lake Erie, but the tops dropped off precipitously as I passed Lake Erie. Again, I got some ice passing through cloud tops while flying at 16,000 feet, the original filed altitude and the one where the ice was predicted to cease in the airmet. I knew there was a lot of ice in the clouds below, but there were no pilot reports, and no pilot on any frequency I used ever mentioned it. Oftentimes shared information, like icing pilot reports, is sparse. I suspect that many pilots deal with ice in their own way and don't want to offer either encouragement or discouragement to others. Also, the temperature rise on the surface of the airplane as speed increases means that a 727's negative ice flight might be an icy one for a Skylane.
The phenomenon that makes the geographic area we have been discussing such an icebox is called "lake effect." The Great Lakes remain relatively warm in relation to the air that moves over them in many weather systems; the result is instability. This causes clouds to build. Because the clouds build rapidly, the supercooled water droplets in them can be large and can remain as a liquid to lower temperatures than in stratus clouds. The ice can be worse toward the top of the clouds, which means if you try to climb out of it, you might almost get to the top but not quite make it because of heavier ice accumuluation up there.
This isn't to belittle the amount of ice found lower in the clouds. A few years ago, a single-engine Cessna left one of the airports on the lee side of Lake Erie and didn't make many miles before the airplane was so ice-laden that it would not fly.
Nothing is true in every case when it comes to weather, but lake- effect clouds behind a cold front often top out in the 15,000- to 18,000- foot level, sometimes lower.
Other areas promote ice formation on airplanes. Mountains are great for this and the bigger the mountain the better. Flying across Arizona one day, I was enjoying a nice flight until we passed through a turbulent cloud that was perched on a ridge, and a great splatter of ice formed on the airplane in a matter of moments. I flew out on the other side, and all was well, but even a deiced airplane would not have lasted long flying a course that was less perpendicular to the ridge than mine. Just as you visualize the flow over mountains in clear weather to anticipate up- and downdrafts and turbulence, apply this to lifting and ice.
With lifting as the key to the really serious stuff, we can imagine some other areas where the ice problems will be greatest. Air swirls around, inward, and upward in a low pressure area. That means lifting. At the altitudes we fly, the interface between cold air and moisture is most likely bad for us to the east and north of the low. This is also where freezing rain will be found, and this is something that absolutely has to be avoided. If you are flying in the warm air above the cold air at the surface, which occurs ahead of a warm front, and there is no ice at your cruising level, but freezing rain is reported at the surface, that is an automatic mandate to divert to an airport where there is no freezing rain. Likewise, if you are flying VFR beneath the clouds and freezing rain starts to fall, the nearest suitable airport is the place to be.
Ice occurs at other times and other places, and wherever it is found, it needs to be avoided. We need to remember a few lessons about icing: It is often said that there won't be much ice at temperatures below minus 15 degrees Celsius. That is true in pure stratus clouds, but it isn't so true where there is rapid lifting or even in the cold tops of stratocumulus clouds behind a cold front.
Virtually all IFR airplanes have some anti-icing capability — in the form of the pitot heat system. This is best turned on whenever in clouds or precipitation or, put another way, in visible moisture. Failing that, turn it on when the temperature drops below 10 degrees C and the airplane might be flying into clouds and precipitation. The point is that pitot heat is so important that turning it on needs to be a reflex action. If it isn't, then consider just leaving it on.
Pilots flying more powerful airplanes often feel that they can climb out of virtually any icing condition, especially if they have deicing equipment. I think that has always worked in jets, but it hasn't always worked in turboprops or turbocharged airplanes. A lot of thrust will get an airplane up and out of harm's way; those of us without a lot of thrust have to know what we could be getting into and do the proper planning.
Finally, the question of deicing or anti-icing equipment: It is a wonderful thing to have, but, especially in light airplanes, flight operations with it should be the same as if it isn't there. Try to stay out of ice, and when it is encountered, do something to get out of it. It is but a tool to use just as you might use a Stormscope or radar to stay out of thunderstorms.
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